The problem of overloading in mechanical power presses has received much attention over a period of many years. Such presses utilize a portion of the energy stored in a flywheel to actuate a mechanical linkage which moves the slide through its work stroke and thereby shapes the material in the die. Because of the excess energy available in the flywheel, mechanical interference encountered during the work stroke of the slide can produce undue stress on the components of the press. Such interference may be due to a variety of causes, among which are the presence of a tool or other foreign item left in the die, improper adjustment of the die, feeding of material which exceeds the thickness for which the die has been adjusted, and many other causes.
As mechanical interference is encountered, the press will attempt to complete the work stroke of the slide utilizing stored energy from the flywheel. Since the stored energy in the flywheel is substantially greater than that which is required for the work stroke, the dissipation of excess stored energy will create destructive stress on the gears and drive linkages of the press. As a further consequence, such interference may also severely damage the die.
Systems which prevent the attempted completion of the work stroke of the press once mechanical interference has been encountered are known in the prior art. The following U.S. patent and publication are illustrative:
U.S. Pat. No. 2,937,733, Issued: May 24, 1960, Inventor: James C. Danly. PA0 Hydraulic Overload Systems--Francis E. Heiberger, (Understanding Presses and Press Operations, pp. 166-170, Copyright 1981, Society of Manufacturing Engineers).
In both of these prior systems, a hydraulic piston and cylinder connection is interposed between the slide and a driving pitman of the press. As mechanical interference is encountered during the work stroke, the force between the pitman and the slide increases, leading to a corresponding increase in pressure inside the hydraulic cylinder. When this pressure exceeds a predetermined value corresponding to the estimated tonnage capacity of the press, a relief/dump valve lowers the hydraulic pressure in the cylinder and a pressure switch is actuated to stop the press.
During the work stroke, the capacity of the press does not remain constant, but varies with the angular position of the eccentric shaft and the corresponding vertical displacement of the slide. At the half-way point of the work stroke, when the eccentric is at a 90.degree. angle with respect to its lower dead center position, the capacity of the press is considerably less than when the eccentric is in the lower dead center position. Since the press capacity varies in this manner, the use of a single estimated value of the tonnage capacity of the press to establish the existence of an overload condition can only provide a first approximation as to when an actual overload condition occurs. If the system uses an estimated tonnage capacity corresponding to the tonnage capacity near the bottom of the work stroke position, mechanical interference encountered by the press before reaching that particular position will necessarily overload the press before the system can detect the overload condition. Moreover, if the estimated tonnage capacity used as a reference for overload is that which would overload the press during the beginning of the work stroke, the control system may indicate the presence of an overload condition when no such condition actually exists.